Energy-efficient CO2/CO interconversion by homogeneous copper-based molecular catalysts

Facile conversion of CO2 to commercially viable carbon feedstocks offer a unique way to adopt a net-zero carbon scenario. Synthetic CO2-reducing catalysts have rarely exhibited energy-efficient and selective CO2 conversion. Here, the carbon monoxide dehydrogenase (CODH) enzyme blueprint is imitated by a molecular copper complex coordinated by redox-active ligands. This strategy has unveiled one of the rarest examples of synthetic molecular complex-driven reversible CO2 reduction/CO oxidation catalysis under regulated conditions, a hallmark of natural enzymes. The inclusion of a proton-exchanging amine groups in the periphery of the copper complex provides the leeway to modulate the biases of catalysts toward CO2 reduction and CO oxidation in organic and aqueous media. The detailed spectroelectrochemical analysis confirms the synchronous participation of copper and redox-active ligands along with the peripheral amines during this energy-efficient CO2 reduction/CO oxidation. This finding can be vital in abating the carbon footprint-free in multiple industrial processes.

The authors have significantly improved the manuscript on their interesting findings on bidirectional/reversible catalysis for CO2/CO interconversion.For publication in Nature Communication, I would still recommend a more quantitative description of the catalytic behavior of their catalysts: 1. Generally the authors assign catalytic performance of their "Complex 1" as reversible while it is not strictly the case for all data described as such.For instance they state: "We have assigned these signals as CO2 reduction and CO oxidation features, respectively, where the catalysis occurs reversibly with minimal overpotential requirements in either direction."However the data shown in figure 1A display a clear change in slope when transitioning from CO2 reduction to CO oxidation showing that catalysis remains bidirectional.Even if the overpotential is low, it is not minimal.The wording "nearly reversible" or better "bidirectional" should be used instead of "reversible for all measurements in organic solvents.On the other hand catalysis in pure aqeous solvents seem in deed very close to reversible as highlighted by the measurements under CO/CO2 (Figure 3E).Adjusting the experimental conditions to avoid mass transport limitation could make this highly convincing (see following point).
2. Generally, the paper lacks quantification of the catalytical potentials for both CO oxidation and CO2 reduction which is complicated by mass transport limitations of the substrate in all catalytic currents (peak shaped currents) shown in the manuscript.Adjusting the catalyst concentration (using less catalyst) vs the substrate concentration under CO/CO2 atmosphere, and adjusting the scan rate of the measurement could help reach a catalytic regime limited by catalysis rather than diffusion to enable reliable quantification of the catalytic properties.The author could check how this was carried out for a reversible catalysts for H+/H2 interconversion (Figure 2 and 3 in Angew.Chem. Int. Ed.2023, e202302779).The data in Figure 3E and 3F in the present manuscript could be good starting points for this (for instance reducing catalyst concentrations at least 10 fold may allow to reach a regime where the catalytic current is limited by catalysis only).This would be highly valuable to support the conclusion of the present manuscript and to compare the catalytic performance with other previously reported reversible/bidirectional catalysts for CO2 reduction or H+/H2 interconversion.
Reviewer #2 (Remarks to the Author): In this work, a copper center combining redox-active ligands unveiled a reversible CO2 reduction and CO oxidation.The inclusion of amine groups around the copper center provides biases toward CO2 reduction in organic/aqueous media.However, I found many are missing at this shape.
1.The copper is saturated in coordination and how could the copper center perform catalysis?However, Cu may leach out to deposite as Cu(0), then there is chance to perform catalysis.
2. Lack of electrolysis data.No product/potential dependence is performed.
4. The amount of CO produced by CO2 reduction during the CV is usually very few, is it enough to generate CO oxidation signals in CV?It is necessary to make a control experiment in CO atmosphere.

Figure S27.
It is unusual to observe C2H4 signal in GC.In order to recognize the source of C2H4 (product of CO2 reduction or impurity), it is necessary to make a contrast experiment in Ar or N2 atmosphere.In this manuscript, the authors mimicked the carbon monoxide dehydrogenase enzyme by combining redox-active ligands around a copper center.This approach allows for reversible CO2 reduction/CO oxidation catalysis, similar to natural enzymes.By including proton-exchanging amine groups in the copper complex, the pathway for CO2 reduction and CO oxidation could be adjusted in both organic and aqueous environments.
In the revision, the authors have pretty much solved the concerns raised by previous reviewers.But I do not think Figure 3A and Figure S12A are the kind of Faradaic efficiency that previous reviewer 3 asked.Another revision would be needed to provide this information.
The authors have significantly improved the manuscript on their interesting findings on bidirectional/reversible catalysis for CO2/CO interconversion.For publication in Nature Communication, I would still recommend a more quantitative description of the catalytic behavior of their catalysts: 1. Generally the authors assign catalytic performance of their "Complex 1" as reversible while it is not strictly the case for all data described as such.For instance they state: "We have assigned these signals as CO2 reduction and CO oxidation features, respectively, where the catalysis occurs reversibly with minimal overpotential requirements in either direction." However the data shown in figure 1A display a clear change in slope when transitioning from CO2 reduction to CO oxidation showing that catalysis remains bidirectional.Even if the overpotential is low, it is not minimal.The wording "nearly reversible" or better "bidirectional" should be used instead of "reversible for all measurements in organic solvents.On the other hand catalysis in pure aqeous solvents seem in deed very close to reversible as highlighted by the measurements under CO/CO2 (Figure 3E).Adjusting the experimental conditions to avoid mass transport limitation could make this highly convincing (see following point).
Response: We appreciate the excellent suggestion from the reviewer.We have now probed the catalytic behaviour of the complex C1 in detail (in organic media) to have a better insight into its CO2 reduction and CO oxidation signals and analyzed the electrochemical signals in the context of reversibility.In this regard, we have prepared different concentration variants of the complex in DMF and recorded its response under a 1:1 CO/CO2 gas mixture.Here, we have recorded the cathodic CO2 reduction signal and anodic CO oxidation signal in a single run.The scan started at the equilibrium potential, and the initial scan direction was at the anodic direction.Here, the background current response was also collected and duly subtracted from the obtained catalytic data beyond the potential where the catalytic current response was noticed.As shown in the following figure, the current response continued to grow beyond the equilibrium potential in either redox direction.background corrected data showcase the presence of both CO2 reduction and CO oxidation at all conditions.At low scan rates, the catalyst showcases bias towards CO2 reduction compared to CO oxidation.
The background corrected data for C2 has now been added in the SI segment with appropriate discussion in the main text.
We have also mentioned the catalytic behaviour as "nearly reversible" in organic solvents as suggested by the reviewer.
2. Generally, the paper lacks quantification of the catalytical potentials for both CO oxidation and CO2 reduction which is complicated by mass transport limitations of the substrate in all catalytic currents (peak shaped currents) shown in the manuscript.Adjusting the catalyst concentration (using less catalyst) vs the substrate concentration under CO/CO2 atmosphere, and adjusting the scan rate of the measurement could help reach a catalytic regime limited by catalysis rather than diffusion to enable reliable quantification of the catalytic properties.The author could check how this was carried out for a reversible catalysts for H+/H2 interconversion (Figure 2 and 3 in Angew.Chem. Int. Ed.2023, e202302779).The data in Figure 3E and 3F in the present manuscript could be good starting points for this (for instance reducing catalyst concentrations at least 10 fold may allow to reach a regime where the Email: arnabdutta@chem.iitb.ac.in arnab.dutta@iitb.ac.inContact: +91-9537995998 4 catalytic current is limited by catalysis only).This would be highly valuable to support the conclusion of the present manuscript and to compare the catalytic performance with other previously reported reversible/bidirectional catalysts for CO2 reduction or H+/H2 interconversion.
Response: We thank the reviewer for this valuable suggestion, and we have executed the experiment of C1 under 1:1 CO2/CO conditions at (A) variable concentrations (0.1, 0.2, and 1.0 mM) and (B) at variable scan rates (20, 50, and 100 mVs -1 ).We have followed the lead provided by the authors in the article for probing DuBois-type catalyst for H2 oxidation/H2 production activity (Angew.Chem.Int. Ed., 2023, e202302779, Proc. Natl. Acad. Sci, 111, 16286-16291, 2014).We have now provided a similar treatment to probe the reversible vs.

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The presence of water is vital in this step as it possibly supports the formation of a new C-O bond while promoting the redox change at the copper center."Itneeds citation of references to support this statement.Reviewer #3 (Remarks to the Author): bidirectional nature (for CO2 reduction/CO oxidation) of C1 under variable conditions.As show in the figure below, C1 demonstrated reversible CO2 reduction/CO ooxidation in DMF when 1:1 CO/CO2 mixture was present.